Ad-Supported Freemium Sprint MVNO RingPlus has long required that customers make an outgoing call at least every 60 days. Customers who fail to meet that requirement are subject to having their plans canceled and their service terminated. The rule makes sense as it costs money to offer free cell phone service and the ring-back ads that play when making a call are one of RingPlus' sources of revenue.
It looks like RingPlus stepped up enforcement of the new rule yesterday. There's an active thread Account Suspended. Old Plan Lost Forever? on the RingPlus Forum where multiple RingPlus users are reporting that their accounts were suspended for not making calls in the last 60 days. There are at least eight users reporting suspensions since yesterday and all of them say that they made at least one call in the last 60 days. The emailed suspension notices say that suspended users have four days to switch to a different RingPlus plan or they will lose their phone number and any cash balance they have. Switching to a new plan will currently cost you anywhere from $2.99 (for the Mad Fun paid plan) to $45 (for the Happy Turkey Day Free Plan if you have a zero balance) plus taxes and fees averaging 20%.
If you you have a RingPlus free plan, I strongly encourage you to check your Dashboard and verify that you have made at least one completed call of one minute or more in the last 60 days. Actually I recommend making a qualifying call at least every weeks or two as it looks like RingPlus' call frequency algorithm is a little trigger happy.